1. Field of the Invention
The present invention relates to an optical information recording medium and a recording method using the same.
2. Description of the Related Art
Conventionally, it is known that a phase can be changed between an amorphous phase and a crystal phase by irradiating a laser beam onto a thin film made of a chalcogen material formed on a substrate. The research and development of a phase-change type optical information recording medium utilizing this phenomenon have been conducted actively.
In the phase-change type recording medium, a laser beam whose output is modulated between two power levels: a recording level and an erasure level are irradiated onto an information track of the recording medium, whereby a new signal can be recorded while an old signal is being erased.
Generally, in such a recording medium, a multi-layer film containing not only a phase-changeable recording layer but also other layers is used as an information layer for recording information. For example, a multi-layer film containing a protective layer made of a dielectric material or a reflective layer made of metal is used as an information layer.
The protective layer made of a dielectric material has the following functions of:
1) protecting a recording layer from mechanical damage from the outside;
2) increasing the rewritable number of times while reducing thermal damage that occurs in the case where a signal is rewritten repeatedly;
3) enhancing a change in optical characteristics by utilizing the interference effect by multi-reflection; and
4) preventing a chemical change due to the influence from the outside air.
As a material for the protective layer achieving the above-mentioned functions, an oxide such as Al2O3, TiO2 and SiO2, a nitride such as Si3N4 and AlN, an oxynitride such as Sixe2x80x94Oxe2x80x94N (JP3(1991)-104038A), a sulfide such as ZnS and a carbide such as SiC have been conventionally proposed. As a material for the protective layer, ZnSxe2x80x94SiO2 (JP63(1988)-103453A) that is a mixture of ZnS and SiO2 also has been proposed. Among these materials, ZnSxe2x80x94SiO2 is used for a commercialized phase-change type optical disk.
One of the reasons why ZnSxe2x80x94SiO2 has been used is as follows: ZnSxe2x80x94SiO2 has a relatively small thermal conductivity among dielectrics, so that it can suppress thermal diffusion during recording to enhance a recording sensitivity. Another reason is as follows: ZnOxe2x80x94SiO2 has a small internal stress, so that cracking is unlikely to occur even if a thick layer is formed using this material, and adhesion with respect to a recording layer is high.
In an optical information recording medium, the recording density can be increased by shortening the wavelength of a laser beam to be radiated. The recording density also can be increased by increasing the numerical aperture of an objective lens for condensing a laser beam to decrease the spot diameter of the laser beam. The recording density also can be increased by recording information onto both grooves for guiding a laser beam and lands therebetween. Furthermore, the recording density can be increased by using a plurality of recording layers. A recording medium provided with a plurality of recording layers and a recording/reproducing method already have been disclosed (JP9(1997)-212917A, WO96/31875, JP2000-36130A). Furthermore, a layer recognizing member and a layer switching member for conducting recording/reproducing by selecting one recording layer from a plurality of recording layers have been disclosed (WO96/31875).
In the optical information recording medium, a general-purpose laser diode typically is used as a light source for recording/reproducing. This makes it necessary to conduct recording in a range of a limited laser output. Therefore, as described above, as a material for a dielectric layer adjacent to a recording layer, ZnSxe2x80x94SiO2 has been used conventionally.
However, in order to record/reproduce a large capacity of information in a short period of time, there is a demand for recording/reproducing at a higher linear velocity. As a linear velocity is increased, a laser irradiation time per unit area is shortened, which makes it necessary to enhance the recording sensitivity of the recording layer. Particularly, in the case of a recording medium including a plurality of recording layers, it is very important to enhance the recording density of the recording layer placed at a position farthest from an incident surface of a laser. In this respect, there is a demand for a material replacing ZnSxe2x80x94SiO2 that has been used conventionally. In order to enhance the recording sensitivity of the recording layer, it is required to use a protective layer made of a dielectric with a thermal conductivity lower than that of a conventional dielectric.
Furthermore, the protective layer made of a dielectric with a low thermal conductivity has an effect of suppressing the phenomenon (so-called cross-erasure) of erasing a part of a mark of an adjacent track by thermal diffusion during recording, and hence, such a protective layer is preferable for increasing a recording density.
Therefore, with the foregoing in mind, it is an object of the present invention to provide an optical information recording medium capable of recording information with satisfactory reliability even at a high linear velocity, a method for producing the same and a recording/reproducing method using the same.
In order to achieve the above-mentioned object, an optical information recording medium of the present invention includes: a first substrate, a second substrate placed in parallel with the first substrate and an information layer (A) placed between the first substrate and the second substrate, wherein the information layer (A) includes a recording layer and a dielectric layer placed so that a distance from the dielectric layer to the recording layer is 20 nm or less, the recording layer is changed between at least two different states that are identified optically by irradiation of a light beam incident from the first substrate side and the dielectric layer contains ZnS and Si as main components. The inventors of the present invention found that recording can be conducted at a lower laser power in the recording medium using a dielectric layer made of Znsxe2x80x94Si (mixture of ZnS and Si), compared with the recording medium using a dielectric layer made of Znsxe2x80x94SiO2. More specifically, by using a dielectric layer made of Znsxe2x80x94Si, a recording medium with a high recording sensitivity can be obtained. Furthermore, Znsxe2x80x94Si is a material that is stable chemically and mechanically. Furthermore, since the film-formation speed by sputtering of Znsxe2x80x94Si is higher than that of Znsxe2x80x94SiO2, Znsxe2x80x94Si is preferable in terms of productivity.
The above-mentioned optical information recording medium may include a plurality of information layers placed between the first substrate and the second substrate, and the information layer closest to the second substrate among the plurality of information layers may be the information layer (A).
In the above-mentioned optical information recording medium, the information layer (A) may include a reflective layer placed on the second substrate side with respect to the recording layer.
In the above-mentioned optical information recording medium, the recording layer may be changed reversibly between at least two different states that are identified optically. In this case, the recording layer may be made of an alloy containing Te and Sb. Furthermore, the recording layer may be made of an alloy selected from the group consisting of a Gexe2x80x94Sbxe2x80x94Te based alloy, a Gexe2x80x94Snxe2x80x94Sbxe2x80x94Te based alloy, an Agxe2x80x94Inxe2x80x94Sbxe2x80x94Te based alloy and an Agxe2x80x94Inxe2x80x94Gexe2x80x94Sbxe2x80x94Te based alloy. Furthermore, the recording layer may be made of a Gexe2x80x94Sbxe2x80x94Te based alloy, and the alloy may contain Ge in an amount of 30 atomic % or more. Furthermore, the recording layer may be made of a Gexe2x80x94Snxe2x80x94Sbxe2x80x94Te based alloy, and the alloy may contain Ge and Sn in an amount of 30 atomic % or more in total. Furthermore, a thickness of the recording layer may be 3 nm to 20 nm.
In the above-mentioned optical information recording medium, the recording layer may be changed reversibly between two different states that are identified optically. In this case, the recording layer may be composed of Te, O and at least one element M selected from the group consisting of Al, Si, Ti, V, Cr, Mn, Fe, Co, Ni, Cu, Zn, Ga, Ge, Zr, Nb, Mo, Ru, Rh, Pd, Ag, In, Sn, Sb, Hf, Ta, W, Re, Os, Ir, Pt, Au and Bi. In this case, a content of oxygen in the recording layer may be 25 atomic % to 60 atomic %, and a content of the element M in the recording layer may be 1 atomic % to 35 atomic %. Furthermore, a thickness of the recording layer may be 5 nm to 70 nm.
Furthermore, in a method of the present invention for recording information onto an optical information recording medium having an information layer including a recording layer changed between at least two different states that are identified optically by irradiation of a light beam, and a dielectric layer containing ZnS and Si as main components, in a case where recording marks with the same length are formed in a first region where recording is conducted at a linear velocity of VL (m/sec.) and a second region where recording is conducted at a linear velocity of VH (m/sec.) larger than VL, recording is conducted by modulating the light beam between a first power and a second power smaller than the first power, and a time TL (sec.) for irradiating the light beam of the first power in the first region, a time TH (sec.) for irradiating the light beam of the first power in the second region, the VL and the VH satisfy a relationship: TLxc2x7VL less than THxc2x7VH.
These and other advantages of the present invention will become apparent to those skilled in the art upon reading and understanding the following detailed description with reference to the accompanying figures.